While there are a few models that can interchange tanks and bowls, most are configured to work optimally as a set as indicated in our Product Showcase. In fact, if you do your own mixing and matching, the toilet may not flush at all! Gravity Fed and Pressure Assisted tanks and bowls cannot be mixed. A 3.5 gallon per flush tank and 1.6 gallon per flush bowl (and vice versa) will not function together, as the trapway designs are completely different based upon the gallons per flush of the entire unit.